In the electrophotographic art, the following methods are commonly known: the direct method, wherein the surface of a photosensitive layer of a photosensitive member is given a charge and subjected to an exposure to form a latent electrostatic image which is developed with a developer to visualize the image, thereupon the visualized image is directly fixed on the photosensitive member in order to provide a copied image; the toner image transfer method, wherein a visual image on a photosensitive member is transferred onto a transfer paper, e.g., paper where the transferred image is fixed to form a copied image; the latent image transfer method, wherein a latent electrostatic image on a photosensitive member is transferred onto a transfer paper where the latent electrostatic image is developed and fixed.
It is conventionally known that inorganic photoconductive materials, such as selenium, cadmium sulfate, zinc oxide and the like, are used as photoconductive materials to form the photosensitive layer of a photosensitive member employed in the electrophotographic methods, mentioned above. These photoconductive materials have diverse advantages; they can be given an electrical charge of a proper potential; the smaller effluence of charge in the dark, and the irradiation to light can rapidly neutralize the charge. Such materials, however, have various disadvantages. For example, selenium photosensitive materials incur greater manufacturing costs and require careful handling, since they are vulnerable to heat and mechanical impacts. Cadmium sulfate photosensitive members and zinc oxide photosensitive members cannot provide either stable sensitivity under the highly humid environment or stable properties for a longer period because a coloring material having been incorporated into the members causes both charge-oriented deterioration due to corona charge and colorfading due to exposure.
On the other hand, various organic photoconductive polymers including polyvinyl carbazole have been already proposed. Compared to the above-mentioned inorganic photoconductive materials, these polymers excel in the properties including coating properties, lightness, but are inferior in sensitivity, durability, and stability to environmental fluctuation.
Low molecule organic photoconductive compounds are preferable since their coating properties or electrophotograhic properties can be regulated by arbitrarily designating the type, composition and the like of a binder being used together with the compounds. However, the combined use with a binder requires that each of the compounds is highly compatible with the binder.
The photosensitive member containing high molecular or low molecular organic photoconductive compounds being dispersed into the resin of binder have disadvantages including a greater residual potential due to the greater traps in the carrier causes poor sensitivity. Accordingly, there have been proposals for blending a charge transporting material with an organic photoconductive compound in order to solve the above-mentioned disadvantages.
Diverse organic compounds have been disclosed as charge transporting materials, however, these compounds have various disadvantages. 2,5-bis(P-diethylaminophenyl)-1,3,4-oxadiazoles, for example, described in U.S. Pat. No. 3,189,447, have a poor compatibility with a binder, and tend to crystalize. Diaryalkane derivatives described in U.S. Pat. No. 3,820,989, though having satisfactory compatibility with a binder, develop deterioration in sensitivity due to repeated use. Hydrozon compounds described in Japanese Patent Publication Laid Open to Public Inspection No. 59143/1979, though having comparatively good residual potential properties, are disadvantageously inferior in sensitivity and charging capacity.
Thus, it is the fact that there are few charge transporting materials having properties practically advantageous in forming a photosensitive member.